The thyroid, a small, butterfly-shaped gland below your Adam's apple, pretty much rules your body, says Douglas Husbands, DC, CCN, a clinical nutritionist and chiropractor in San Carlos, California. It regulates your metabolism, explains Husbands, and an underactive thyroid (hypothyroidism) slows everything down - from your pulse and temperature to your energy level and the rate at which you burn calories. An overactive thyroid (hyperthyroidism), on the other hand, revs the body into overdrive.

According to Mary Shomon, author of Living Well With Hypothyroidism (HarperCollins), "As many as 59 million Americans have a thyroid condition. Unfortunately, the vast majority do not get diagnosed."

Hypothyroidism - the more common condition by far - usually crops up in women, people older than 60, and those with a family history of thyroid problems. Worldwide, the most common cause is iodine deficiency, but in the US and other developed countries where people use iodized salt, the principal culprits include autoimmune disorders (in which immune cells attack the thyroid tissue), radiation to treat cancers of the head and neck, and medications such as lithium. The laundry list of potential symptoms includes fatigue, forgetfulness, depression, heavier periods, dry hair and skin, mood swings, weight gain, intolerance to cold, hoarseness, and constipation.

If you have a number of these symptoms, you should think about having your thyroid hormone (thyroxine or T4) and thyroid-stimulating hormone (TSH) levels tested. Depending on the results, you may need to take daily synthetic thyroid hormones. But you can also complement hormone therapy with the following key nutrients, all of which are necessary for healthy thyroid function. Even if you have normal thyroid levels, these essential vitamins and minerals can help keep your thyroid running smoothly.

Selenium. Before thyroid hormones can do their job, your body must convert them into an active form. This requires the mineral selenium. Husbands recommends that people with thyroid problems - as well as those with healthy glands - supplement with selenium (he prefers selenomethionine) daily to promote thyroid health. You can get the same amount from one big handful of Brazil nuts, which are by far, the best food source of the mineral.

Zinc. You also need an adequate amount of zinc for your thyroid hormones to function correctly, notes Shomon. In a small clinical study last year, researchers at the University of Massachusetts found that the thyroid hormone levels in zinc - deficient women improved significantly after four months of treatment with a daily dose of zinc. Although further research is needed, Shomon suggests because zinc can block copper absorption, be sure to take copper daily as well.

Iodine. Essential for many bodily functions, iodine also plays a role in the production of thyroid hormones. "But it’s tricky," says Shomon, "because while iodine deficiency is a major risk factor for hypothyroidism, too much iodine can aggravate the thyroid and worsen existing conditions." Iodine is a trace mineral should be taken in microgram levels (mcg).